Many devices and systems include various numbers and types of sensors that perform various monitoring and/or control functions. Advancements in micromachining and other microfabrication techniques and associated processes have enabled manufacture of a wide variety of microelectromechanical (MEMS) devices. In recent years, many of the sensors that are used to perform monitoring and/or control functions have been implemented into MEMS devices.
One particular type of MEMS sensor that is used in various applications is an accelerometer. Typically, a MEMS accelerometer includes, among other component parts, a proof mass that is resiliently suspended by one or more suspension springs. The proof mass moves when the MEMS accelerometer experiences acceleration. The motion of the proof mass may then be converted into an electrical signal having a parameter magnitude (e.g., voltage, current, frequency, etc.) that is proportional to the acceleration.
Another type of MEMS accelerometer that is used to sense acceleration is commonly referred to as a teeter-totter capacitive acceleration transducer, or a “teeter totter accelerometer.” A typical teeter totter accelerometer includes an unbalanced proof mass suspended over a substrate using a fulcrum or other axis. The proof mass forms first and second capacitors with a first and a second conductive electrode, both of which are formed on the substrate. During an acceleration perpendicular to the substrate the proof mass tilts to a degree that is proportional to the acceleration, and the gap between the proof mass increases on one side of the axis, and decreases on the opposite side of the axis. The capacitances of the first and second capacitors change in opposite directions, and the capacitance changes are detected and used to determine the direction and magnitude of the acceleration.
Teeter-totter accelerometers are generally simple and cost-efficient to manufacture. However, since the proof mass tilts rather than moving in a uniform manner with respect to an opposing electrode, the change in the average gap size between the proof mass and the opposing electrode is relatively small. The small change in average gap size sometimes translates to a suboptimal capacitance change for some purposes. Since the change in the average gap size is relatively small, teeter-totter accelerometers may not be adequately sensitive to small accelerations. Further, the base capacitance (capacitance under zero acceleration) is often lower than desired, as making the plates larger results in larger die area and hence higher production costs.
Also, if a MEMS device such as one of the above-described MEMS accelerometers experiences a relatively high acceleration or is exposed to a relatively high force, the proof mass can move beyond a desired distance. In some instances, such movement can potentially damage the MEMS device. Moreover, the MEMS device can exhibit unstable behavior if the proof mass and/or other portions of the MEMS device travel too far when a voltage is supplied to the MEMS device. Thus, many MEMS devices include one or more types of travel stops or motion limiters that are arranged to limit the movement of the proof mass and/or other portions of the MEMS device.
Although presently-known devices and methods for limiting the travel of MEMS device components are generally safe, reliable, and robust, these devices and methods do suffer certain drawbacks. For example, some capacitor structures include integral travel stops within an active capacitor region. Some typical travel stops include corrugations or dimples that are formed with or on a proof mass, and arranged to fit between the proof mass and another capacitor plate to prevent the two masses from making electrical or mechanical contact. Other common travel stops are not part of the proof mass, but are separate structures that are positioned in an active area between a proof mass and a capacitor plate. Because the travel stops are located in an active area, they include a dielectric or other nonconductive layer that contacts the proof mass to impede the proof mass motion. Over time, the nonconductive layer can wear or lose its ability to isolate the proof mass and the capacitor plate, thereby shortening the functional life of the MEMS device.
Accordingly, it is desirable to provide a MEMS accelerometer that is highly sensitive, has a high base capacitance, and also consumes minimum die area. In addition, it is desirable to provide a MEMS accelerometer that is not prone to damage resulting from impacts involving the functional components. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.